1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copying machine and a laser printer.
2. Description of Related Art
In the art of electrophotographic copying apparatus, an electrophotographic copying apparatus which comprises a refeeding unit to feed the same sheet twice for duplex copying (images are formed on both sides of a sheet) and a composite copying (images are formed overlaid on one side of a sheet) is provided. This type of refeeding unit is mainly composed of an intermediate tray for containing a sheet temporarily and a refeeding roller and so on. Also, the refeeding unit has to be adjusted such that a refed sheet comes in transporting standard position at a sheet transporting way of the copying apparatus.
Conventionally, a refeeding unit frame which houses the intermediate tray is fixed to a body frame of the copying apparatus at a specified position, and the sheet transporting standard position of the refeeding unit is adjusted by turning a position of a widthwise sheet aligning plate which is provided at the intermediate tray. However, in this structure, the refeeding unit has to be pulled out from the body of the copying apparatus for adjusting the sheet transporting standard position, and that shows a poor working efficiency.
In the duplex copying, sheets which have obtained an image on one side are contained on the intermediate tray one by one, and then refed. Generally, the sheets which are piled up on the intermediate tray are refed from the most bottom sheet. This is called a "bottom sheet refeeding".
In the bottom sheet refeeding, the sheets on the intermediate tray have to be aligned accurately. Otherwise, while refeeding, one or more sheets are drawn by the most bottom sheet and refed together, or the sheets on the intermediate tray can not be fed from the most bottom sheet in correct order, which may cause a sheet jam.
Since the transporting way become longer, a sheet may be transported to the intermediate tray askew or may be put on the intermediate tray out of position. Also, the sheet transported to the intermediate tray is heated while going through a fixing device and curled. Thus, the sheets piled up on the intermediate tray are not aligned enough to be refed. Therefore, particularly in a high-speed copying apparatus, above-mentioned malfunctions while refeeding are apt to occur.
Also in the bottom sheet refeeding, the sheet is refed by a frictional force between the refeeding roller and the most bottom sheet. Therefore, when the number of sheets on the refeeding roller is small, enough frictional force for refeeding can not be obtained. In this case, the sheets are pressed by a sheet pressing member made of a metal plate from above. Thereby, a pressure is given between the refeeding roller and the sheet, and a necessary frictional force is generated. On the other hand, when a sheet is coming onto the intermediate tray, the sheet pressing member which is provided above the intermediate tray have to retreat in order to make the sheet transporting way. For this purpose, the sheet pressing member is movable between a retreating position and a pressing position.
In a conventional refeeding unit, the sheet pressing member is movable between a retreating position and a pressing position. However, since the moving distance of the sheet pressing member is large, the sheet pressing is carried out with a significant delay. In addition, noise is made when the sheet pressing member comes down and beats the sheet. Moreover, while refeeding, if the sheet pressing member keeps staying at the pressing position, the sheets piled up on the intermediate tray are drawn to the refeeding direction by the frictional force between the fed sheet and the sheets piled up, and a loop may be formed at a front end of the sheet in the refeeding direction. This loop has to be eliminated for a smooth refeeding.
Also in the bottom sheet refeeding, the weight of the sheets is applied to the most bottom sheet. Therefore, when the most bottom sheet is refed, a pick up pressure and a frictional force between the sheets are changed according to the number of piled sheets. Thus, while refeeding, two or more sheets may be refed together, or the sheets on the intermediate tray may not be able to refed from the most bottom sheet in correct order, which may cause a sheet jam.
Further, while a copying operation, since the sheets are transported to the intermediate tray successively, the jam which occurred on the intermediate tray can not be found easily. Thus, in the conventional copying apparatus, a photosensor is provided at the transporting way near the transporting roller which sends the sheet to the intermediate tray, in order to detect the jam at the transporting way. However, in this method for detecting the jam at the transporting way near the transporting roller, it is impossible to detect a jam on the intermediate tray. When a jam occurred on the intermediate tray, a sheet which is transported after the jammed sheet is interrupted and jammed at the transporting way. In this method, the jam on the intermediate tray can be detected indirectly by detecting a jam of a sheet which is transported after the above-mentioned jammed sheet at the transporting way. Therefore, when the copying apparatus is stopped upon the jam detection, a plural number of sheets are stuck on the intermediate tray. That may damage parts around the intermediate tray or make the removal of jammed sheets difficult.